Wow thanks so much for all your input everyone. I have printed off the Tube directions... Looks like we'll do that rather than the HEX. @mcott - we will be flying from YOW-YYZ, and then on to LHR. Not direct unfortunately, but that's fine.
@uktravelbear thanks so much for the link to the London Pass. We will do that, it looks a lot cheaper. I'm really interested in the Thames River Cruise...

Does London have any hop on/hop off tours? We were also looking at those. I saw that it's offered (online), so maybe we'll do that as well.. Still have to research it a little more, but I'm very excited. Never been to the UK before. I hope the weather will be nice